1. Field of the Invention
The invention relates to electrophoretic displays, in particular its electro-optic switching particles preparation method and the devices made by the method.
2. Background of the Relevant Art
An electrophoretic display has benefits in terms of several information display application fields such as sun light readability, power saving nature with its display image memory function with display medium itself, wide viewing angles, and good compatibility with current available display system both in terms of display image signal sources and interfaces.
On the other hand, current known electrophoretic display technologies have some downside in terms of competitiveness against current commercially available flat panel display technologies. One of the drawbacks is inconsistency of memory type and full motion video image capability in terms of drivability of the display media. Since, display media's image memory effect is very effective to save power, on the other hand, full motion video images require continuous rewriting of the screen, and display media's image memory effect is even obstacle function for the continuous rewriting. In principle, display media's image memory function is avoidable function for full motion video image reproduction. As long as continuous and consecutive rewriting is required, display media's power saving with its memory effect does not have contribution to power saving at all. Moreover, refreshing scheme of the stored image requires both more power and writing time, resulting in inconsistency between power saving and full motion video image capability.
The other drawback is somehow limited display resolution of current available electrophoretic display system. There are several reasons why current known electrophoretic display systems have some limitation in their image resolution, specifically for color image reproduction. Detail of this particular technical limitation will be discussed below. In spite of above beautiful performance of electrophoretic display systems, above inferior issues compared to current commercially available flat panel display technologies have made limitation of electrophoretic based display technologies in terms of penetration or creation of their market.
Therefore, it is highly expected for an electrophoretic display technology to eliminate any drawbacks that are not seen in current commercially available flat panel display system, and to keep the beautiful nature of electrophoretic display technologies such as sun light readability, a paper like image readability, consistent power saving capability in general display environment, and so on.
Current available electrophoretic display technologies and/or current known electrophoretic display technologies are featuring their display media's own image memory capability. A display medium's own image memory capability is very beneficial with display module power saving. Not only ambient light reflection basis of the display technology, but also no need of continuous and sequential image refreshing provides significant power saving. A display medium's own image memory function is also very good to provide very stable and firm image to a viewer. Unlike continuous refreshing type of screens which are current most of flat panel displays use including CRTs (Cathode ray Tubes), LCDs (Liquid crystal Displays), OLEDs (organic Light Emitted Diodes), memorized completely still image provides very stable and non-jittering images just same with paper based images.
In spite of a great benefit of stable and very soft image for human eyes of memorized image on an electrophoretic display, display medium's image memory function significantly restricts a motion video image capability regardless optical switching response speed of a switching particle of the electrophoretic display. Current widely being used flat panel display technologies described above do not have display media's own image memory effect. This means, once changed optical state by externally applied stimulation such as electric field application, goes back to the previous state before the external stimulation was applied immediately after the external stimulation is removed. Therefore, it is required to continuous pumping with so-called refreshing excitation to keep the image regardless still images and motion video images for such display technologies. Although these display technologies require continuous refreshing which consumes a certain power, automatic return to the original or initial optical state can eliminate any “re-set” driving scheme. Without “re-set” driving scheme, in particular a large display information content screen case, it helps writing up time of whole screen significantly shorter. Without “re-set” driving scheme, even display medium's optical switching time is relatively longer such as 100 ms; it is possible to have some level of motion image with some image artifacts such as non-crisp motion video image. On the other hand, even an optical switching display medium's switching time is very fast such as 10 ms, if it requires “re-set” driving scheme, actual full screen writing time is far longer than required motion video image reproduction. Therefore, in order to have a full motion video capable electrophoretic display without sacrificing significant power saving for still image application, both having a display medium's display image memory capability for still image purpose, and non-display medium's display image memory capability for motion video image are necessary. None of current known electrophoretic display technologies provide such duplicated technical capabilities, therefore, it is highly expected to realize these somewhat inconsistent performances to eliminate drawback that is not seen in current major flat panel display technologies.
As one of very intrinsic natures of electrophoretic display technologies, their display resolution is dependent optical switching particle size. In order to have good enough light scattering performance which is very important for paper like display image, the optical switching particle size must satisfy mu-light scattering that means the particle size must be large enough compared to visible light wavelength (normally it is about 0.6 micron). On the other hand, if the particle size is too large such as over 10 micron, the display resolution has significant limitation to avoid parallax issue as a display device. As FIG. 1 illustrates, if switching particle size is say 30 micron, and the display image pixel pitch is 30 micron, the display panel gap (the definition of the panel gap is the layer thickness of display medium sandwiched by two electrodes substrates) should be over 30 micron in order to secure switching particles well enough movement in a limited space. In actual display panel, due to surface topography and panel manufacturing process tolerance requirement, the required panel gap is most likely over 40 micron. In this case, as FIG. 1 illustrates, some of incident light from ambient light condition is scattered next pixel area's optical switching particles, resulting in wrong pixel image. In order to avoid this parallax problem, if the switching particle size is 30 micron, pixel pitch should be over double of panel gap. This condition requests pixel pitch of over 80 micron with panel gap of 40 micron. Based on current most popular color display reproduction means that is use of micro-color filters, sub-pixel corresponding to each primary color sub-pixel size should be larger than 80 micron. As is well known with reflective subtract color system, in order to have well enough light reflectivity, not only subtract primary colors of cyan, magenta, and yellow, white sub-pixel, and sometime, even black sub-pixel are implemented. Based on this sub-pixel primary color filter method, single full-pixel size must have over 320 micron or even 400 micron pitch. This image resolution pitch is far larger than current printed image or even current mobile application LCDs cases.
When an optical switching particle size is small enough, for example smaller than 5 micron, this particle size would be well enough to keep light scattering with visible wavelength. In such smaller optical switching particle cases, in order to have multi-color or full-color display systems, following three ways are currently known.
(1) Primary colored multiple particles
(2) Micro color filters with small sized black and white particles
(3) Primary colored separated particles
For the case (1), each primary colored switching particle such as cyan, magenta, and yellow colored particles have different threshold voltage to respond externally applied electric field, and different threshold selectively drive each colored particle in a panel, resulting in reproducing multi-color image with good enough display resolution such as smaller than 20 micron pixel pitch. This case is introduced by Naoki Hiji, et. al., “Novel Color Electrophoretic E-Paper Using Independently Movable Colored particles” paper number 8.4, SID 2012 Digest page 85 to 87 (2012) in Boston. This method effectively realize multi-color and/or full-color electrophoretic display image with fine enough pixel resolution. However, this method requires each colored particle's display image memory effect and at least three duplicated driving scheme to differentiate each primary colored particle's selective driving. This driving scheme significantly limits total screen writing time, resulting in significant difficulty of full motion video image capability regardless each particle's switching time. In short, this particular method is very effective to have a fine enough pixel pitch, on the other hand, this method significantly restricts full motion video image reproduction capability. Therefore, this method still keeps significant drawback to compete with current available flat panel display technologies.
For the case (2), as long as small enough size of switching particles have fast enough optical switching as well as no particular display image memory function at the display image, in conjunction with micro color filters that is very popular and widely used for current most of flat panel display technologies, it works to provide fine enough image with multi-color and/or full color image reproduction with full motion video reproduction. However, this method has significant technical difficulty in manufacturing. Even 30 micron sub-pixel pitch micro-color filter fabrication is nowadays very popular manufacturing. This means it is well enough conceivable as a volume manufacturing product. On the other hand, a micro color filter method has a general liability in terms of limited light reflectivity due to its nature of light absorption. In order to maximize light reflectivity, a back-side on color filter configuration illustrated in FIG. 2 is normally applied. This configuration also requires a transparent electrode formation under the micro color filter as shown in FIG. 2 in order to avoid surface reflection of transparent electrode which significantly degrades color purity of reflected light. Since a transparent electrode such as ITO (substantially, it consists of Indium Oxide) has very high reflective index such as over 3.0, then, if color filter is formed under the Ito electrode, most of incident light is reflected on the surface of ITO, and could not reach at color filter, resulting in very poor color purity. Therefore, this system needs to form color filter on the ITO. Each color filter includes each color pigment, and each pigment contained color filter works as a different dielectric layer under the two parallel electrodes. Due to nature of electrophoretic displays, each different dielectric layer stacked pixel including switching particles layer provides different threshold to each color pixel switching particles. This variation of threshold voltage makes gray scale control significantly difficult, or even taking into account temperature dependence of dielectric characteristics, it is almost impossible to control gray scale by threshold voltage means.
For case (3), as long as each colored switching particle is prepared small enough size, in principle, it is fine to provide fine enough pixel pitch. However, as a display panel preparation, this method requires clear separation of each colored switching particles depending on each pixel. If the display medium consists of fluid or liquid shape, only conceivable way to avoid color mixing is physical separation of each colored pixel. In this case, due to requirement of physical separators at each color sub-pixel, significant area on the screen is sacrificed in terms of aperture ratio. Significant reduction of aperture ratio results in significant reduction of reflectivity on the display screen. Film type of display medium or self-sustained display medium may avoid necessity of physical separator on the display substrate. However, in this case, very fine pitch patterned each primary colored particles are formed in the self-sustained display medium in the scale rang of micron scale. Under the consideration of film medium's size accuracy, micron scale of distance accuracy may not be easy. Even the film is prepared with very accurate manner, at the panel fabrication process, more or less; two substrates give some mechanical pressure to the “soft” self-sustained display medium, resulting in change of accurately aligned particles in the film.
As discussed above, current known technologies have significant technical challenges in terms of providing practically fine enough pixel image that is already established as volume manufacturing technology in LCDs, OLEDs and so on. Therefore, new electrophoretic displays are doomed to provide fine enough pixel pitch as well as well enough controllable multi-color, and/or full color full motion video images.